Known tubes, for example hot cathode and cold cathode low pressure mercury discharge tubes, have a phosphor coating on their internal surface which, when subjected to a gas mercury vapour discharge rich in ultra violet radiation provide a light output in the visible spectrum.
The current coating method employed in manufacturing the tubes involves mixing an organic binder, for example nitro cellulose and ethyl cellulose, in a solvent or blend of solvents such as xylene or butyl acetate. When the solution obtains a pre-determined viscosity phosphor is blended into it and the resulting suspension is used to coat the inside of the glass tube by up flush or down flush methods. The coating is dried by passing air through the tube to remove the solvent and when fully dry the tubes are heated to a high enough temperature to combust the organic binder in the presence of air, oxygen, or air enriched with oxygen so that all the organic carrier material is burnt off leaving a phosphor coating on the tube held in place by an inorganic binder.
This method is disadvantageous in that it requires the input of a considerable amount of energy to burn off the organic materials and often results in damage to the glass tubes as a result of the elevated temperatures to which they are subjected during the burn-off operation. Additionally the organic materials are lost during the burning off process.
Known glass plate displays also have disadvantages in respect of their coatings. In these displays a printing medium including phosphors and a binder is screen printed onto the glass but prior printing media are disadvantageous in that the phosphor particles are only loosely bound to the glass by the binder and consequently coated glass plates must be handled with particular care, especially when it is borne in mind that the coated surface is exposed and consequently prone to damage. Additionally, if any organic material is intermixed with the phosphors it decomposes during the exposure, in use, to high intensity ultra-violet radiation and has the effect of discolouring the phosphor and reducing the light output. Separate areas printed with different phosphors to give different coloured light may also intermingle giving an illuminated effect of poor quality.
It is an object of the present invention to obviate or mitigate these disadvantages.
In the present specification the term fumed silica is intended to refer to a colloidal product produced by the hydrolysis of silicon tetrachloride vapour in a flame of hydrogen and oxygen at approx. 1800.degree. C. The term aluminum oxide C is intended to refer to a product produced by flame hydrolysis of anhydrous aluminum chloride.